You grow, plants grow, and yes—rocks grow too! They just grow very slowly.
The earth is made up of different rocky plates, kind of like a big jigsaw puzzle. Together, all of this rocky jigsaw puzzle is called the lithosphere. Plate tectonics is the study of these rocky tectonic plates and how they move and change. Plates move around in different ways. Sometimes they push together at a convergent boundary. At these boundaries, volcanoes and mountains form as two chunks of rock push together. At transform boundaries, the plates slide past each other. Sometimes the sliding isn’t very smooth, and earthquakes happen. At divergent boundaries like mid-ocean ridges, the plates move away from each other and new rock oozes up from underneath, adding to the sea floor.
In this experiment, you’re going to model what happens when the ocean floor spreads.
Create a model of the spreading sea floor.
- Cardboard cylindrical container
- White 8 ½ x 11 in. piece of paper
- Colored pencils
- Take a cardboard, cylinder-shaped container (such as a Quaker Oats container) and cut a vertical slit about 3 ½ inches long and ¼ inch wide down the side. The slit is your mid-ocean ridge, the place where the plates are moving away from each other.
- Cut a piece of white paper in half lengthwise.
- On each piece of paper, measure two inches in from the end and fold the paper so that there is a section on each end to hold onto.
- Measure inward another two inches from the fold, and color in that two-inch wide strip.
- Continue to measure in two-inch segments, coloring every other section.
- Place the unfolded ends of the paper into the slit in the container. Holding them by the folds, pull the pieces of paper out again. If you imagine that the slit is the midocean ridge where the plates are moving away from each other, the paper is the new liquid rock coming out from the ridge. The first bit of paper to come out is the oldest rock, and the last section of paper to come out is the youngest. Imagine that your paper goes on forever. Soon, the first bit you took out of the hole will be far away from the midocean ridge. Of course, “soon” in geological time is a very long time in human terms!
- Take your pieces of paper and tape the ends that aren’t folded to the pencil.
- Put the pencil inside of the container and pull the ends of the pieces of paper up through the slit.
- Twist the pencil one way, and the papers will move out and away from each other. This is what happens at a divergent plate boundary on the mid-ocean ridges.
- Twist the pencil the other way, and the papers will move in and toward each other. This is what happens at a convergent plate boundary. Imagine what would happen if those papers had bumps on them. They’d get all bunched together at the hole, and create mountains.
How does seafloor spreading work? Imagine that you’re baking a really delicious chocolate cake. After some time in the oven, the top of the crust begins to crack and the pieces of the cake’s top move away from each other. Unfortunately, you’ve made the batter a little too wet, and the cake underneath is not yet cooked. As the top pieces of the cake crack and move away from each other, the gooey underside of the cake moves up into the crack, pushing the pieces of the cake’s top crust away from each other.
The hard crust of the cake is the lithosphere. Underneath the hard part of the earth is the asthenosphere, the gooey liquid rock that sits underneath the hard outer crust. Sea floor spreading happens at places where plates are moving away from each other and where the liquid rock from the asthenosphere can come up to the lithosphere. In the places where the plates are moving apart, magma (liquid rock) moves up into the cracks and solidifies, making a new ocean floor, just like cake batter would ooze up through the cracks.
One intriguing thing about the rock that comes from seafloor spreading is that it shows the history of the Earth. For example, every million years the Earth’s magnetic poles tend to reverse 4 or 5 times. South becomes north and north becomes south. The rocks on the seafloor show the history of the magnetic changes in the earth. They are magnetized according to wherever the pole was at the time the rock formed and cooled on the sea floor. This allows scientists to understand the magnetic history of the earth and the history of these rocks. You illustrated this magnetic shift when you colored in every other two-inch section.